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Steve: Hi, Steve Mirsky here with a short episode of Science Talk. As you may know, PBS has been running a four-part nova series with physicist, Brian Greene, called "The Fabric of the Cosmos." The last of the four parts is airing tonight, November 23rd. After part one debuted, Greene hosted a live Q&A here in New York City. The first question dealt with something that's been a big story in science news of late, the question of the alleged faster-than-light neutrinos. As you listen, there's a bonus a couple of backstage people who apparently didn't know that their electronic communication was bleeding into the microphone feed. Enjoy!

Questioner: And here is one from Twitter. You spoke about the speed of light in the program, but what about neutrinos going faster than the speed of light?

Greene: Well, that happened after the program was made. No, it's a great question because some of you may have heard there's this interesting recent result, where neutrinos were sent from Geneva through the Earth's crust to Gran Sasso in Italy, a 450 mile, race track, if you will; and the thing is the measurement seemed to show that the neutrinos are reaching their destination 60 nanoseconds before they should, namely they're getting there 60 nanoseconds even before a beam of light would be able to travel that distance. The neutrinos seemed to be going faster than the speed of light. This would be a remarkable discovery, but this is a very difficult experiment. There are many, many uncertainties. Even the researchers who put forward this result know that there are uncertainties. You have to measure that distance precisely, you have to know exactly the length of time for that journey, the moon pulls on the Earth's crust and that can slightly change distances, there are all sorts of complications. These are very smart people, so maybe all of these uncertainties have been taken into account, but I would bet everything I hold dear—well, almost everything I hold dear—that the result won't hold up to scrutiny. And I do say almost everything, because there's a small chance that my two little kids may be watching online right now. So, Alec and Sevilla, Daddy learned his lesson last time! (laughter) I will not bet on you this time around, so you don't need to worry about that. But I should say that if the result turns to be correct, it would be thrilling. I hope it's correct. This is what we live for, this kind of a revolution. I don't think this is one of those moments, but it could be. Okay, questions here—anybody? I'm looking around for somebody who has a mike, anybody have a mike? Right there.

Questioner: [If neutrinos did go faster than light, could you explain] what that means in case if that is confirmed?

Greene: What could it mean if neutrinos did go faster than the speed of light, if special relativity, this idea that Einstein gave us is wrong? Well first, it's useful to know why is that we think that neutrinos wouldn't go faster than the speed of light. Well, Einstein's theory shows that as an object is moving, the faster it goes, the more mass the more energy it has, the more mass means that it's harder to push it, even further. So, if a neutrino is approaching the speed of light, it gets heavier and heavier, needs an ever stronger push; and his math seems to show that if the neutrino went right to the speed of light, you need an infinite push to make it go faster still, and an infinite push isn't something that can happen. And that's why we don't think this is a correct result. If it is, well we'd have to rethink a lot of our basic understanding of relativity. The most naïve interpretation of the result would suggest that you might be able to send a signal to the past. So if something can go faster than the speed of light, if I have a neutrino gun and I fire it at you, the neutrino, in some way of thinking about it, would hit you before I pull the trigger. And that is so weird, that's so bizarre, that we really would need independent confirmation of those results to believe them. And people are undertaking experiments in the next few months, so we'll see whether independent experiments do confirm this result. I don't think they will, but I hope that I'm wrong.

Steve: If you missed "The Fabric of the Cosmos" when it aired, don't fret, the episodes stream in their entirety at the PBS Web site, http://www.pbs.org. And if you want to watch them on your bigger TV screen, check your listings for what will surely be numerous repeats. For Scientific American's Science Talk, I'm Steve Mirsky. Thanks for clicking on us.